1. Field of the Invention
This invention relates to a zoom lens, and an imaging apparatus using this zoom lens as an image taking lens, and particularly to a rear focus type zoom lens suitable for a small-sized imaging apparatus such as a digital still camera, a home use video camera, and the like, and also capable of performing a zooming rate of 3 to 5 times, and an imaging apparatus using such zoom lens.
2. Description of the Related Art
In recent years, a digital still camera and a digital video camera have widely spread as home use apparatuses, and further a miniaturized design has been required to those small-sized imaging apparatuses. On this account, an image taking lens to be mounted, particularly a zoom lens is required to be miniaturized in its total length and shortened horizontal depth. Further, an improvement in the lens performance is also required for such image taking lens for a digital still camera use with reference to an increase of the number of pixels in such imaging device in addition to the miniaturized design.
It has been known, for example, that so-called rear focus type zoom lens, where a lens groups other than a first lens group which is provided at the most object side are moved to focus, are made possible to easily miniaturize a total lens system, and to obtain an imaging performance suitable for a solid state imaging device having a larger number of pixels. As such rear focus type zoom lens, a zoom lens having 5 lens groups is well known, wherein the zoom lens is configured to have a first lens group having a positive refracting power, a second lens group having a negative refracting power, a third lens group having a positive refracting power, a fourth lens group having a positive refracting power, and a fifth lens group having a negative refracting power from object side in this order, and a zooming operation is carried out by moving the second and fourth lens groups, and a focusing operation is carried out by moving the fourth lens group. For example, in the Patent Document 1 mentioned later, there is disclosed a zoom lens which satisfies following Equations (1) to (3), wherein focal lengths of the third lens group and the fifth lens group are f3, and f5, of the fifth lens group at a position where an object distance is at infinity, a focal length of the second lens group is f2, and focal lengths in this total system at a wide-end and a tele-end are fw, and ft, respectively.0.8<|f5/f3|<2.1  (1)1.2<|β5|<1.6  (2)0.25<|f2/√{square root over (fw·ft)}|<0.37  (3)
Recently, it has been considered to delete a projected portion of lens system upon an image taking by folding an optical path in the middle from the first lens group to an image plane, by shortening the length of the lens when assembled in the imaging apparatus, and by setting a movable direction of the lens upon zooming operation in the up and down directions. For example, such zoom lens that comprises a first lens group having a positive refracting power, a second lens group having an negative refracting power, a third lens group having a positive refracting power, and a fourth lens group having a positive refracting power from an object side in this order is known as disclosed in a Patent Document 2 as mentioned below. That is, this conventional zoom lens is configured to include a lens construction having 4 lens groups that performs a zooming operation by moving the second and fourth lens groups, wherein the first lens group includes a first lens of a single lens having an negative refracting power, a prism for bending an optical path, and a second lens of a single lens having a positive refracting power from the object side in this order.
Patent Document 1: JP 3015192
Patent Document 2: JOP 2000-131610
By the way, in the zoom lens having the optical system where the optical path is folded by a prism, it is possible to realize further miniaturization and a low-profile model design by miniaturizing a size of the prism. However in the zoom lens disclosed in the Patent Document 2, there is a problem that if a diameter and a thickness of lenses included in the first lens group is made small, it is difficult to further miniaturize the prism because of the deterioration of the optical performance.